Asymptomatic and symptomatic oropharyngeal disorders can lead to an inability to swallow or to difficulty in swallowing. These disorders may be caused, for example, by neurodegenerative diseases, strokes, brain tumors or respiratory disorders.
Swallowing is a complicated action whereby food is moved from the mouth through the pharynx and esophagus to the stomach. The act of swallowing may be initiated voluntarily or reflexively but is always completed reflexively. The act of swallowing occurs in three stages and requires the integrated action of the respiratory center and motor functions of multiple cranial nerves, and the coordination of the autonomic system within the esophagus. In the first stage, food or some other substance is placed on the surface of the tongue. The tip of the tongue is placed against the hard palate. Elevation of the larynx and backward movement of the tongue forces the food through the isthmus of the fauces in the pharynx. In the second stage, the food passes through the pharynx. This involves constriction of the walls of the pharynx, backward bending of the epiglottis, and an upward and forward movement of the larynx and trachea. Food is kept from entering the nasal cavity by elevation of the soft palate and from entering the larynx by closure of the glottis and backward inclination of the epiglottis. During this stage, respiratory movements are inhibited by reflex. In the third stage, food moves down the esophagus and into the stomach. This movement is accomplished by momentum from the second stage, peristaltic contractions, and gravity.
Although the main function of swallowing is the propulsion of food from the mouth into the stomach, swallowing also serves as a protective reflex for the upper respiratory tract by removing particles trapped in the nasopharynx and oropharynx, returning materials to the stomach that are refluxed into the pharynx, or removing particles propelled from the upper respiratory tract into the pharynx. Therefore, the absence of adequate swallowing reflex greatly increases the chance of pulmonary aspiration.
In the past, patients suffering from oropharyngeal disorders have been subjected to dietary changes or thermal stimulation treatment to regain adequate swallowing reflexes. Thermal stimulation involves immersing a mirror or probe in ice or another cold substance and stimulating the tonsillar fossa with the cold mirror or probe. Upon such stimulation, the patient is directed to close his mouth and attempt to swallow. While dietary changes and exercise rehabilitation using thermal stimulation may be effective for treating oropharyngeal disorders, some patients may require weeks or months of therapy. It is also difficult to distinguish patients who require such treatments from patients who recover spontaneously.
Muscle fibers are generally characterized as Type I or Type II, depending on their contraction rate, endurance, resistance to fatigue and other characteristics. Type I muscle fibers are characterized by slow contraction rates, high endurance, slowness to fatigue and low power. In contrast, Type II muscle fibers are characterized by fast contraction rates, low endurance, quickness to fatigue and high power. All muscles contain both types of fibers, and several of the muscles involved in swallowing contain a higher proportion of Type II fibers. It is believed that the high speed and dynamic and forceful action of the swallow are due to this preponderance of Type II fibers.
Most conditions treated in therapy are characterized by a degree of disuse atrophy. Disuse atrophy refers to changes in the muscle after a period of immobilization or reduced activity. The most obvious change is a decrease in the cross-sectional area of the muscle belly, with Type II fibers being affected to a greater degree than Type I fibers. Swallowing musculature shows these typical changes with disuse, but the impact on these muscles is relatively great since the overall percentage of Type II fibers is higher. During exercise rehabilitation, Type I fibers are contracted first, whereas the larger sized Type II fibers are involved only when the effort increases. Consequently, Type I fibers receive the most benefit from exercise rehabilitation. On the other hand, during electrical stimulation, Type II fibers are the first to contract, whereas Type I fibers contract only later when the pulse width and intensity are raised above a certain threshold. Consequently, electrical stimulation preferentially trains Type II fibers.
Neuromuscular electrical stimulation (NMES) has been used to alleviate pain and stimulate nerves, as well as a means for treating disorders of the spinal cord or peripheral nervous system. Neuromuscular electrical stimulation (as well as electrical muscle stimulation) has further been used to facilitate muscle reeducation and with other physical therapy treatments. In the past, neuromuscular electrical stimulation or electrical muscle stimulation were not indicated for use in the neck because of concerns that the patient could develop spasms of the laryngeal muscles, resulting in closure of the airway or difficulty in breathing, and/or because of concerns that the introduction of electrical current into the neck near the carotid body would cause bradycardia and consequent hypotension.
More recently, neuromuscular electrical stimulation has been used to stimulate the recurrent laryngeal nerve to stimulate the laryngeal muscles to control the opening of the vocal cords to overcome vocal cord paralysis, to assist with the assessment of vocal cord function, to aid with intubation, and other related uses. Generally, there have been no adverse reactions to such treatment techniques. However, neither neuromuscular electrical stimulation nor electrical muscle stimulation have been used in the treatment of oropharyngeal disorders to promote the swallowing reflex, which involves the integrated action of the respiratory center and motor functions of multiple cranial nerves.
It would be desirable if a simple, non-invasive method and device could be provided for treating oropharyngeal disorders and promoting swallowing in an effective manner within a relatively short treatment period.